Method and apparatus for scanning a key or button matrix

ABSTRACT

An improved key matrix scanning technique conducts a first pre-scan on a first array of connections in a key matrix to identify any activated keys or buttons associated with the first array. A second sub-scan is conducted on a second array of connections in the key matrix but only for the connections in the first array that are detected as having activated keys.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/888,666, filed on Jul. 8, 2004, now issued as U.S. Pat. No.7,158,056, the disclosure of which is herein incorporated by reference.

BACKGROUND

FIG. 1 shows how a typical keyboard matrix 12 is constructed. Two thinsheets 14 and 16 of plastic or polymer material have printed conductivetraces 18 and 20, respectively. The traces 18 and 20 are arranged suchthat when pressure is applied at certain locations, such as when keys 22on a keyboard are pressed, one trace on sheet 14 makes an electricalconnection with one trace 20 at the corresponding location on the othersheet 16.

Typically, the sheets 14 and 16 are separated by a third sheet 24 withno printed traces. The third sheet 24 includes holes 26 aligned with thepositions of buttons 22. When no pressure is applied to the area abovethe holes 26, the conductive traces 18 and 20 do not make contact. Whenone of the keys 22 is pressed, one of the traces 18 on sheet 14 ispushed through the corresponding hole 26 making an electrical contactwith one of the traces 20 on sheet 16. The electrical cross-connectionbetween the trace 18 and the trace 20 is associated with a particularkey or button 22. The information associated with the identified key orbutton 22 is forwarded to a computer, processor, or other computingdevice.

For clarity, only a small number of row and columns traces are shown inFIG. 1. The traces 18 and 20 on the two printed sheets 14 and 16 arearranged so that each trace 18 on the upper sheet 14 crosses each trace20 on the lower sheet 16 over a hole 26 only once. In this way, eachbutton 22 on the keyboard will make a single unique contact between oneof the traces 18 on the upper sheet 14 and one of the traces 20 on thelower sheet 16. Typically, one sheet has eight traces referred to as keymatrix rows and the other sheet has between 16 and 24 traces referred toas key matrix columns. In this way, an 8×16-24 key matrix is formed,with each button 22 corresponding to a single cross-connection point inthe matrix.

Generally, a metal sheet (usually connected to electrical ground) isplaced underneath the bottom sheet 16 in order to provide mechanicalrigidity and to help prevent electrostatic discharge events. Since theresistivity of the conductive ink used to print the traces 18 and 20 isquite high, the printed traces are quite wide, typically in the order ofaround 2 millimeters (mm) wide.

Due to the size of most keyboards, the traces 18 and 20 are also quitelong, particularly the row traces which may “zig-zag” back and forthacross the length of the keyboard in order to cross each of the columntraces. Lengths of over 0.5 meters (m) are common. Because of therelatively high resistivity, the impedance of the traces 18 and 20 maybe 10 ohms per square (s), or up to a couple of hundred ohms. Thecombination of the trace width and length, the thinness of the polymersheets 14 and 16, and the proximity of the metal grounding sheet createsa significant capacitance in the traces 18 and 20.

Typically, keyboards contain a microcontroller having Input/Output (I/O)pins connected to the row and column traces 18 and 20. Themicrocontroller applies signals to the I/O pins and senses the signalson other I/O pins to detect actuation of the keyboard buttons 22.Digital logic or firmware is typically used to execute this process.This arrangement is also used in applications other than keyboards, forexample remote controls, where the state of many keys must be detected.

The firmware or logic senses which, if any, buttons 22 are being pressedstarts by first connecting a pull-up resistance between each of the rowtraces and a high voltage, for example 5 volts (V). Typically thisresistance is contained within an I/O cell of a Microcontroller Unit(MCU) pin connected to the row traces. The MCU I/O pins connected to thecolumns are typically held in a high-impedance state. A first column,such as column 0 in FIG. 2 is then driven with a low voltage. The MCUthen reads the logic state of the 8 rows for column 0. If all the rowshave a logic “1” state, then the firmware infers that no buttons arepressed on the first column 0 of the matrix.

If one or more of the rows have a logic “0” state, the MCU infers thatthe buttons are pressed corresponding to the cross-connection locationsin the matrix corresponding to column 0 and the rows having the logic 0state. The column 0 is returned to the high impedance state and theprocess is repeated for each of the remaining columns 1-n. The MCUperiodically repeats this entire process driving each column to a knownlogic state and then scanning each row for the known logic state.

The MCU then also conducts some post processing for any detected keypresses. For example, simultaneously pressing three keys located in aright-angled triangle shape relative to each other in the key matrixwill cause a fourth “phantom” key press to be detected. The firmwarepost-processes the list of detected key presses and executes analgorithm to remove any possible “phantom” keys from its list of pressedkeys.

The pull-up resistances need to be significantly higher than theimpedance of the longest trace so that when one end of the trace isgrounded, the output of the voltage divider formed by the trace andpull-up resistor is a low voltage. Typically, pull-up resistances arearound 10 thousand (k) ohms.

Because of the significant capacitance of the traces 18 and 20, andrelatively high pull-up resistances, the time constant of the formedResistance/Capacitance (RC) is typically around 10 microseconds (us).After a column pin is driven low, it is therefore necessary to monitoreach cross connection in the key matrix for a significant amount of time(t) as shown in FIG. 2. Sometimes this time delay needs to be as much as100 us to ensure the state of the row I/O pins correctly reflect thestate of the associated keys 22 (FIG. 1).

The time required to scan an entire scan 8×20 key matrix used in atypical Personal Computer (PC) keyboard is therefore the time (t)multiplied by the number of columns (n). The total scan time istherefore (n×t) and can be as much as 2 milliseconds (ms). In a normalwired keyboard, where power is provided by the computer attached to thekeyboard, this is not a significant problem. However, in battery poweredkeyboards, particularly wireless keyboards, it is desirable to maximizebattery life.

In order to detect and filter key bounce, the key matrix is typicallyscanned every 10 ms, For a 2 ms total scan period, the MCU thereforeperforms the matrix scanning algorithm as much as 20% of the time thekeyboard is in active use.

A typical wireless keyboard consumes an average of 2-3 milli Amps (mA)when in active use. To save energy, the MCU is placed in a low powersleep mode when not actively scanning the matrix, processing the resultsof each scan, and transmitting data associated with key presses (ifany). The typical current use by the MCU in this type of application isaround 3 mA when active (i.e., executing instructions). However, thecurrent use is negligible while in the sleep mode. Thus, the averagecurrent attributable to the process of scanning the matrix is around 600uA (20% of 3 mA). This amounts to 20-30% of the overall currentconsumption of the keyboard.

These existing methods of keyboard matrix scanning are slow, consume toomuch power, and prevent the MCU from performing other tasks for asignificant period of time while scanning the matrix. The presentinvention addresses this and other problems associated with the priorart.

SUMMARY OF THE INVENTION

An improved key matrix scanning technique conducts a first pre-scan on afirst array of connections in a key matrix to identify any activatedkeys or buttons associated with the first array. A second sub-scan isconducted on a second array of connections in the key matrix but onlyfor the connections in the first array that are detected as havingactivated keys.

The foregoing and other objects, features and advantages of theinvention will become more readily apparent from the following detaileddescription of a preferred embodiment of the invention which proceedswith reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a key matrix.

FIG. 2 is a timing diagram showing how the key matrix in FIG. 1 isscanned.

FIG. 3 is a block diagram showing an improved key matrix scanningsystem.

FIG. 4 is a more detailed diagram of the circuitry used for conducting afirst pre-scan.

FIG. 5 is a diagram of the circuitry in FIG. 4 configured for conductinga second sub-scan.

FIG. 6 is a block diagram showing how the circuitry in FIGS. 4 and 5conduct the pre-scan and sub-scan.

FIG. 7 is a timing diagram showing how the scan time is reduced for theimproved key matrix scanning.

FIG. 8 is a system diagram showing one example of an application for theimproved key matrix scanning.

DETAILED DESCRIPTION

FIG. 3 shows an improved key matrix scanning system 30. The scanningsystem 30 can increase battery life of a wireless keyboard and othertypes of remote control and battery operated devices by reducing thetime required for a Microcontroller Unit (MCU) 32 to scan a key matrix34.

The MCU 32 can be any type of programmable processing device. In otherembodiments, the MCU 32 is firmware such as a Programmable Logic Device(PLD) that is hard coded to perform the operations described below. Thekey matrix 34 can be any circuitry used for sensing depression of keysor buttons on a user control device. The terms keys and buttons are usedinterchangeably in the description below and refer to any type ofactuation device used for communicating a user input to a computingdevice.

Typically only one button 22 is pressed at a time on a keyboard orkeypad used with the key matrix 34. In other cases, two keys 22 arepressed at the same time. Only in rare cases, amounting to a negligibleproportion of the time that the key matrix 34 is in use, are more thantwo key or buttons 22 pressed at the same time.

The MCU 32 quickly determines during a pre-scan routine 33 whichcolumns, if any, of the key matrix 34 have active keys (e.g., depressedkeys). Rather than scanning all columns, the MCU 32 conducts a secondsub-scan 35 only for the columns identified with active keys during thepre-scan 33. If only one column is detected with active keys during thefirst column pre-scan, then the row sub-scan is conducted only for thatone identified column during the second sub-scan. If two keys aredetected as pressed during the pre-scan, then the second sub-scan isconducted only for the two columns detected with pressed keys.

This is an improvement over current scanning techniques that scan therows of the key matrix for every column. The time taken to determinewhich columns have pressed keys is approximately the same amount of timerequired to scan a single column. Thus, if only one key is detected asdepressed during the pre-scan, the time taken to scan a 20-column matrixis reduced by 90%. If two keys are detected as depressed during thepre-scan, the time taken to scan the matrix is reduced by 85%.

FIG. 4 shows one example of a configuration used for the scanning system30 of FIG. 3 for conducting the first pre-scan operation 33. The keymatrix 34 is shown as an array of columns connections 42 and rowconnections 44. However, it should be understood that this is forillustrative purposes and the two overlapping etch patterns forconnections 42 and 44 may not physically be aligned in columns and rows.The intent is to illustrate that the connections 42 and 44 overlap andthe cross-connections between connections 42 and 44 are created bydepression of a key or button. For example, cross connection 37 iscreated when a key 22 located above the intersection between row 2 andcolumn 1 is depressed. The pins 40 on the MCU 32 or connected to columnconnections 42 and pins 38 are connected to row connections 44.

In the pre-scan configuration in FIG. 4, the MCU 32 configures the pins40 as inputs. Pull-up resistors 36 are connected to columns 42, pullingthe columns 36 to logic high states. The MCU 32 configures I/O buffers39 so that the pins 38 operate as outputs. It should be understood thatthe pull-up resistors 36 can be internal to the MCU 32 or can bediscrete interface circuitry that is separate from the MCU 32.

During the pre-scan, the MCU 32 generates logic low signals 46 on thepins 38 and monitors the signals received on the pins 40. In thisexample, the MCU 32 detects a key or button 22 is depressed atcross-connection location 37 in the key matrix 34 when the logic lowsignal 46 is detected on the column 1 connection. Since there are noother pressed keys, the other column connections 0 and 2-n remain in alogic high state during the pre-scan operation.

FIG. 5 shows the configuration of the scanning system 30 for the secondsub-scan operation 35. The MCU 32 configures the I/O buffers 50connected to the pins 40 as outputs and configures the I/O buffers 39(FIG. 4) connected to pins 38 as inputs. In this example, the MCU 32connects pull-up resistors 48 to the pins 38 pulling all of the rows 44to known logic high states. The MCU 32 then generates a logic low (“0”)pulse only on the columns connections 42 where a logic 0 pulse waspreviously detected during the pre-scan operation in FIG. 4.

In this example, a logic low signal 46 was only detected on column 1 inFIG. 4. Therefore, a logic low signal 54 is only generated for columnconnection 1 in FIG. 5. The MCU 32 scans the rows 44 for the logic lowsignal 54. Since there is only one cross-connection 37 established inthe key matrix 34 by depressed key 22, the logic low signal 54 will onlybe detected on row 2. The MCU 32 determines that the key 22 associatedwith row 2 and column 1 has been depressed and sends the information toan associated host device. The MCU 32 has determined which key has beendepressed without having to generate logic signals 54 on columnconnections 0 and 2-n and then having to scan all the rows 44 for eachcolumn 0 and 2-n.

It should be understood that this is just one example and it is notnecessary that the pins 38 and 40 have to be connected to pull-upresistors or that logic zero signals have to be asserted on the rows andcolumns. Alternatively, pull-down resistors could be used and logic 1values asserted on the column and row connections.

Further, the pull-up or pull-down resistors may not necessarily beinternal to the MCU 32. In alternative embodiments, the resistors may belocated in separate circuitry from the MCU 32. In another embodiment,the logic in the MCU 32, I/O buffers 39 and 50, and the resistors 36 and48 are partially or all implemented in different combinations ofintegrated circuitry and discrete logic circuitry.

Referring to the flow diagram in FIG. 6 and FIGS. 4 and 5, the MCU 32 inblock 60 drives all row connections 44 to logic 0 states with signals 46and connects pull-up resistors 36 to the column connections 42 (FIG. 4).The MCU 32 waits some delay period to allow for charging or dischargingof trace capacitance. For example, the MCU 32 may wait for around 100us. The MCU 32 in block 64 then conducts the pre-scan operation byreading the logic state of each column connection 42. If all columnshave a logic 1 state in block 66, no keys are pressed, and the rest ofthe process is skipped. The MCU 32 returns and waits for the nextpre-scan operation.

If a logic low state is detected on a column connection 42 in block 66,the MCU 32 in block 68 begins the sub-scan operation by connecting thepull-up resistors 48 in FIG. 5 to pins 38. The first column detectedwith a logic 0 signal during the pre-scan operation is driven low at theMCU I/O pins 40 in block 70. The MCU 32 reads the logic state for all ofthe row connections 44 in block 72. The MCU 32 infers that a button isdepressed corresponding to the location, such as cross-connectionlocation 37 in FIG. 5, corresponding to the column and the row detectedwith logic 0 states. The column driven low in block 70 is returned to ahigh impedance state in block 74. The MCU 32 then repeats the operationin blocks 70-74 for each of the other columns, if any, where a logic 0state was detected in block 66. The MCU 32 creates a list of pressedkeys that is then post-processed in a manner similar to conventionalkeyboard processing.

FIG. 7 shows how the overall scan time is reduced by first conductingthe column pre-scan. In the traditional matrix scanning scheme, thereare as many delays as there are columns, regardless of how many keys arepressed. FIG. 7 shows an example where all the columns are scanned forthe first logic low signal 46 applied to all rows 44. The time requiredto scan all columns 42 during the pre-scan operation is (t).

In this example, only column 1 was detected with a logic low signal.Therefore, only one row scan is required for the logic low signal 54applied to column 1. This single row scan for column 1 operation onlyrequires a single delay time t. Thus, the total scan time for theexample shown in FIG. 7 is (2*t).

The improved scanning technique therefore reduces the number of delaysto 1+(the number of columns with keys pressed). Typically the time takento perform each operation is much less than the delay time. Thus, for a20-column key matrix 34, the scan time is reduced by 95% when no keysare pressed, 90% when one key is pressed, 85% when two keys are pressed. . . etc. Only if 20 keys are pressed, and each key is on a differentcolumn, will the scan time increase, and only by 5%.

When no keys are pressed, the MCU 32 will generally be in a sleep modeand is not scanning the key matrix 34. Before going into the sleep mode,the MCU 32 may perform the pre-scan operation in blocks 60-64 (FIG. 6)and enable an interrupt on a low-going transition on all I/O pinsconnected to matrix columns 42. Therefore, if any key is pressed, aninterrupt is generated causing the MCU 32 to wake up from the sleep modeand start the scanning operations.

The “no keys pressed” condition will generally only be detected once perkeystroke, immediately after the last key pressed is released. The mostfrequent scan result will be one key pressed. The second most commonwill be no keys pressed; the third most common will be two keys pressed,and three or more keys pressed is unusual.

FIG. 8 shows a wireless keyboard 90 that includes the MCU 32 thatconducts the improved key matrix scan. The wireless keyboard 90 includesa keyboard 98 that includes keys or buttons that are depressed by auser. The key matrix 34 is contained underneath the keyboard 98. The MCU32 periodically scans the key matrix 34 and identifies any depressedkeys on keyboard 98 as described above.

The MCU 32 sends the key information to a wireless transmitter 94 thatincludes an antenna 96 that wirelessly transmits the depressed keyinformation 100 to some sort of computing device 92. The computingdevice 92 than takes some sort of appropriate action in response to theinformation represented by the key information 100.

In one example, the pre-scan operation reduces the time spent scanningthe key matrix 34 by around 95%. This reduces the average currentconsumption attributable to scanning from 600 micro Amps (uA) to 30 uAfor the example described above. In the case of a wireless keyboard withan average active current of 2.5 mA, the power reduction is around 23%.This translates into around a 30% increase in battery life.

Alternative Embodiments

Wireless keyboards 90, remote controls, and other keypad devices are themost common applications where it would be advantageous to reduce thekey matrix scan time. However, there are other cases where reduced scantime is also useful. For example, in a wired keyboard with some combinedother functions, such as a Universal Serial Bus (USB) keyboard hub. Insuch cases, the MCU 32 has other operations to perform in addition toscanning the key matrix and transmitting the keyboard data. These otheroperations may take up most of the MCU processing power, either all thetime or when processing an event related to the other functionality. Inthese cases, reducing the time spent scanning the key matrix 34 frees upthe MCU 32 for performing other processing.

The improved key matrix scanning operation greatly reduces MCU scantime. This reduces the overall average power consumption since the MCUcan spend a greater proportion of time in a low power sleep mode. Theimproved scanning system in one application reduces power consumption ofWireless USB keyboards, such as keyboard 90 in FIG. 8, improving itscompetitive position over wired keyboards.

The system described above can use dedicated processor systems, microcontrollers, programmable logic devices, or microprocessors that performsome or all of the operations. Some of the operations described abovemay be implemented in software and other operations may be implementedin hardware.

For the sake of convenience, the operations are described as variousinterconnected functional blocks or distinct software modules. This isnot necessary, however, and there may be cases where these functionalblocks or modules are equivalently aggregated into a single logicdevice, program or operation with unclear boundaries. In any event, thefunctional blocks and software modules or features of the flexibleinterface can be implemented by themselves, or in combination with otheroperations in either hardware or software.

Having described and illustrated the principles of the invention in apreferred embodiment thereof, it should be apparent that the inventionmay be modified in arrangement and detail without departing from suchprinciples. I claim all modifications and variation coming within thespirit and scope of the following claims.

1. A method comprising: conducting a pre-scan of a connection-matrixhaving rows and columns that are connectable upon activation of one ormore keys or buttons, wherein the pre-scan is capable of identifyingeither a row or a column of the connection-matrix that corresponds to anactivated key or button; and when at least one row or column isidentified in the pre-scan as corresponding to the activated key orbutton, conducting a sub-scan of the identified row or column to locatea particular row-column connection that corresponds to the activated keyor button.
 2. The method according to claim 1 wherein the pre-scan scansthe columns of the connection-matrix to identify a column of theconnection-matrix that corresponds to an activated key or button, andthe sub-scan scans rows based on the column identified from the pre-scanto locate the activated key or button.
 3. The method according to claim1 further comprising: driving a pre-scan signal on the rows; monitoringthe columns for the pre-scan signal; and identifying any column detectedwith the pre-scan signal as having the activated key or button.
 4. Themethod according to claim 3 further comprising: driving a sub-scansignal on at least one column identified with the activated key orbutton; detecting the sub-scan signal on at least one of the rows; andlocating the activated key or button by associating the detectedsub-scan signal with a key or button in the connection-matrix.
 5. Themethod according to claim 3 further comprising: driving a sub-scansignal on the column identified with the activated key or button;detecting the sub-scan signal on at least one of the rows of theconnection-matrix; and associating the detected sub-scan signal with thekey or button in the connection-matrix.
 6. The method according to claim1 further comprising: connecting each of the columns to a knownimpedance state; applying a first known voltage to each of the rows;sensing the voltage on the columns that do not have a voltage applied;generating a list of columns that have the first known voltage;connecting each of the rows to the known impedance state; applying thefirst known voltage only on the columns identified in the list; andsensing the voltage on each of the rows to identify the activated key.7. The method according to claim 6 further comprising: applying theknown impedance state through a pull-down or pull-up resistance; andapplying the first known voltage having a different logic value than thefirst known impedance state.
 8. The method according to claim 1 furthercomprising: placing a microcontroller or logic in a low power sleepmode; waking up the microcontroller or logic to initiate the pre-scan;identifying the activated key or button according to the pre-scan andthe sub-scan; and placing the microcontroller or logic back into the lowpower sleep mode.
 9. The method according to claim 1 further comprising:conducting the pre-scan and sub-scan in a wireless keyboard; andwirelessly transmitting any identified pressed keys or buttons to a hostprocessing device.
 10. The method according to claim 1 furthercomprising: configuring the rows as outputs and the columns as inputsduring the pre-scan; and configuring the columns as outputs and the rowsas inputs during the sub-scan.
 11. A device comprising: processingcircuitry configured to conduct a first scan of a connection-matrix toidentify either a row or a column of the connection-matrix correspondingto an activated connection, and to conduct a second scan of the row orcolumn identified in the first scan to identify a particular key orbutton in the connection-matrix that corresponds to the activatedconnection.
 12. The device according to claim 11 wherein the processingcircuitry comprises logic circuitry that during the first scan putscolumns of the connection-matrix in a first high impedance logic stateand applies a second logic state to the rows of the connection-matrix.13. The device according to claim 12 wherein the logic circuitry duringthe second scan puts the rows of the connection-matrix in the highimpedance logic state and applies the second logic state only to thecolumns identified in the first scan as having activated connections.14. The device according to claim 12 wherein the logic circuitrycomprises pull-up or pull-down resistors.
 15. The device according toclaim 11 wherein the processing circuitry comprises bi-directionalconnections coupled to the rows and columns of the connection-matrix,the bidirectional connections operating as inputs to the rows andoutputs to the columns during the first scan and operating as outputs tothe rows and inputs to the columns during the second scan.
 16. Thedevice according to claim 11 wherein the processing circuitry is in awireless keyboard that comprises a wireless keyboard transmitter to sendkey or button information to a computer associated with the keyboard.17. The device according to claim 11 wherein the processing circuitry isconfigured to enter a sleep mode for a period of time and thenperiodically wake up to conduct the first and second scans.
 18. A methodcomprising: identifying either a row or a column in a connection-arraythat corresponds to an activated key or button; and sub-scanning theidentified row or column in the connection-array to locate the key orbutton in the connection-array that is activated.
 19. The methodaccording to claim 18 further comprising: pre-scanning columns of theconnection-array to identify the column in the connection-array thatcorresponds to the activated key or button, wherein the sub-scanningscans rows that corresponds to the activated key or button based on thecolumn identified from the pre-scan.
 20. The method according to claim19 comprising: driving a pre-scan signal on the rows of theconnection-array; and monitoring the columns of the connection-array forthe pre-scan signal, wherein any column detected with the pre-scansignal corresponds to an activated key or button.